1. Field of the Invention:
This invention relates to a color CRT used as a television set or a computer terminal.
2. Description of the Related Art:
Cathode-ray tubes (CRTs, hereafter) are used for television sets or various kinds of computer terminals. In order to form an image on the screen, a CRT has an electron gun in an envelope maintaining a high vacuum of 10.sup.-8 Torr. The electron gun emits an electron beam through slits in a shadow mask to an RGB fluorescent screen on a front glass, the electrons hit the screen which emits designated colors, and forms an image on the screen. When considering the avoidance of implosions, a sphere is a favorable shape for CRTs because they are made of glass which reduces the cost of production and enables easier production. As a screen, on the other hand; flat is the most favorable shape. Therefore, one of the most important points in designing CRTs is how to coordinate the two conflicting factors.
In the following, Conventional color CRTs are explained with figures.
FIG. 5 is a side view of a conventional color CRT, a part of which is cut out to show its inside. In this figure, the CRT 1 consists of a panel 2, a funnel 4 adjacent to the panel 2, and a neck 5 which is adjacent to the funnel 4 and has an electron gun inside (not shown in the figure). The panel 2 includes a panel screen 2a whose inner surface is coated by a fluorescent screen 3, and a panel skirt 2b. The panel skirt 2b is sealed with the funnel 4 by frit glass, which is a kind of solder glass having thermal plasticity, at a sealing part 6. Inside the CRT 1, a shadow mask 10 is positioned facing the fluorescent screen 3. In order to avoid deformation of the shadow mask 10, it is fixed to a frame 11 by a stud pin 9 attached to the panel skirt 2b, through a plate spring 13 that is fixed to the frame 11. The CRT as a whole is attached to a device body (not shown in the figure) by fixtures 7 located on the four corners of the panel skirt 2b.
FIG. 6 is a front view of the CRT 1 viewing from the panel 2. Here, X, Y, and Z axes are defined as shown in the figure and the diagonal lines of the fluorescent screen 3 is defined as P axes. As shown in the figure, each of the panel and the fluorescent screen forms a shape quite similar to a rectangle. The ratio of the length of the longer side of the fluorescent screen and the length of the shorter side of the fluorescent screen is usually approximately 4:3.
Since the inside of the CRT 1 is kept at a high vacuum, the direction of deformation is inward. With respect to this type of deformation, we have to separately consider the strength of the two sides of the glass, the outside (against atmospheric pressure) and the inside (against the vacuum). The strength of the glass against the vacuum depends on its compression strength and is negligible, since it is far greater than the tensile strength of the glass. Therefore, only the strength of the external surface of the glass is considered in the following explanations.
FIGS. 7 through 9 show the stress distribution of a CRT. FIG. 7 shows the stress distribution along the Y axis of FIG. 6. In this figure, a solid line SA indicates the cross section of the CRT 1 along the Y axis, and a dashed line 21 shows the stress distribution at the surface of the CRT 1. The figure shows that the critical area covers from an edge of the screen of panel 2 to the sealing part 6, when the inside of the CRT 1 is a vacuum.
FIGS. 8 and 9 show the stress distribution along the X axis cross section and the P axis cross section respectively. In the figures, the solid lines LA and DA show the cross section of the CRT 1 along the X axis and the P axis respectively. The dashed lines 22 and 23 show the stress distribution at each cross section based on the surface of the CRT 1. These figures show that the critical point of tension of CRT as a glass vacuum container is around the area from an edge of the panel screen 2a, through the panel skirt 2b, to the funnel 4. Therefore, these parts should be reinforced by, for example, increasing the thickness of the glass.
By increasing the thickness of the glass, however, the weight of the CRTs also increases, which is undesirable. In order to eliminate the shortcoming, methods such as Japanese Patent Laid Open No. Hei 2-86033 are proposed.
FIG. 10 shows a side view of a conventional CRT, a part of which is cut out to show its inside. This number attached to each part is identical to that of FIG. 5 and explanation is omitted where appropriate. In the CRT, a part of the envelope (the area from the panel skirt 2b to the funnel 4) is made of metal (metal part 8). On the inner surface of the metal part 8, there is a stud pin 9 which is inserted into a positioning hole of a plate spring 13 which is fixed to the frame 11 of a shadow mask 10. In CRTs having this type of configuration, the metal part 8 is more than ten times as strong as glass, and the weight is significantly reduced even if we account for the difference in the specific gravities of the two materials.
However, because the stud pin 9 is made of metal, the electric potential of the metal part 8 and the fluorescent screen 3 becomes equal to that of the shadow mask 10, and in practice a high voltage of, for example, 28 kV is applied. This could cause a dangerous electrical shock when producing or repairing image receiving tubes having CRTs. Therefore, the metal part 8 should be covered by insulation film, but the area of the metal part 8 is so large that uniform covering with the insulation film is quite difficult.
Another method for avoiding electrical shock is to make the fluorescent screen 3, the metal part 8 and the shadow mask 10 be at an equal electric potential, while making the metal part 7 be at an earth potential and the cathod of the electron gun be at a potential of -28 kV. This method, however, is not realistic because the circuit would have to be totally redesigned.